This invention relates generally to electromagnetic clutches, such as for use in controlling the transmission of power from an automobile engine to a refrigerant compressor in an automobile air conditioning refrigerating system, and in particular, to an improved connecting structure between an armature plate and a hub for transfering the rotation of a pulley to a shaft of the compressor.
The general structure of the electromagnetic clutch for use between the automobile engine and the compressor is disclosed in U.S. Pat. Nos. 3,044,594 and 3,082,933. The electromagnetic clutch disclosed in these patents comprises a pulley 1 rotatably supported on a tubular extension of a compressor housing through a bearing 2, as shown in FIG. 1. The pulley is rotated by a belt from the automobile engine. The pulley 1 is provided with a magnetic material in which one or more concentric arcuate or annular slits 3 are formed to define a plurality of concentric annular magnetic pole pieces. A drive shaft 4 of the compressor extends through the tubular extension. A hub 5 is fixed to the terminal end of the drive shaft 4. An annular armature plate 6 is joined by a plurality of leaf springs 7 to the hub 5 in such fashion that the armature plate 6 faces the annular concentric pole pieces with an axial air gap therebetween. A magnetic coil 8 is mounted on the tubular extension to supply magnetic flux for attraction of the armature plate 6 to the magnetic pole pieces.
Thus, when the magnetic coil 8 is energized, the drive shaft 4 is rotated together with the pulley 1 by the engine output, and when the magnetic coil 8 is not energized, the pulley 1 is rotated by the engine output but the compressor is not driven.
In this construction, the armature plate 6 is joined by a plurality of leaf springs 7 to hub 5 which is fixed to drive shaft 4. Therefore, if the armature plate 6 is attracted to the pulley 1 by the magnetic flux, it impacts against the pulley 1, and an impact sound is caused, because the two parts are formed of metal. This impact sound is very annoying to passengers within the automobile.
Moreover, as the compressor is driven, the noise caused within the compressor is transferred through the drive shaft 4 to a stop plate which is part of a magnetic clutch and would be diffused external of the compressor unit by the stop plate.
Furthermore, the impact force occurs at the moment when the armature plate connects the axial end surface of the pulley and begins to suddenly transmit the rotational force. The impact force is a result of the axial motion of the armature plate which places it in contact with the pulley and of the sudden initiation of rotational movement of the armature plate because of its coupling to the rotating pulley. However, this impact force is not cushioned by the rigid connection of the leaf springs between the armature plate and the drive shaft. The sudden torque change is thus transmitted to the drive shaft of the compressor. The leaf springs are disposed to an angular offset to be drawn or pulled generally along their lengthwise dimension in the direction of the rotation. Therefore, if the clutch is rotated in a direction opposite to the normal direction of rotation, shearing stress is caused in the leaf spring. The endurance of the clutch is therefore reduced.
In order to avoid these disadvantages, a holder plate 9 is fixed on the axial end surface of the armature plate 6 at an opposite side of the armature plate 6 than that which faces the pulley 1 by a plurality of rivets, as shown in FIG. 2. An inner end portion of the holder plate 9 and outer end portion of the hub 5 are formed with concentric annular axial flanges 91 and 51, and a rubber member 10 is disposed in the clearance between the axial flanges 91 and 51. Therefore, armature plate 6 is connected to hub 5 through holder plate 9 and rubber member 10. The transmission of sudden torque changes to the drive shaft is thus eased by the rubber member. However, easing of the impact force is dependent on inducing of bending stress and/or shearing stress in the rubber member. Therefore, the rubber member fatigues by the repeated intermittent operation of the clutch and the rubber member becomes cracked or is destroyed.